Insight into the Nanoscale Mechanism of Rapid H2O Transport within

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article 2

Insight into the nanoscale mechanism of rapid HO transport within graphene oxide membrane: the impact of oxygen functional group clustering Shuai Ban, Jing Xie, Yajun Wang, Bo Jing, Bei Liu, and Hongjun Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08824 • Publication Date (Web): 14 Dec 2015 Downloaded from http://pubs.acs.org on December 15, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Insight into the nanoscale mechanism of rapid H2O transport within graphene oxide membrane: the impact of oxygen functional group clustering Shuai Ban,∗, ‡,§ Jing Xie,‡,§ Yajun Wang,‡ Bo Jing,† Bei Liu,¶,† and Hongjun Zhou‡ †State Key Laboratory of Offshore Oil Exploitation, Beijing 100027, China ‡Institute of New Energy, State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Fuxue Road 18, Beijing 102249, China ¶State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Fuxue Road 18, Beijing 102249, China §Contributed equally to this work E-mail: [email protected]

Abstract Realistic models of graphene oxide membranes were developed and validated to interpret the exceptional water permeation in association with XPS, TG-DSC and DVS measurements. With respect of the GO oxidization level, surface distributions of functionalized domains were analyzed in line with TEM observations, and 3 types of interlayer domains in slit pores of GO membranes were identified. The hydrophilicity degrees of as-defined domains strongly influence their H2 O uptake capacities. Calculated sorption enthalpies and isotherms are in good agreement with experimental data, and the results indicate the dominant role of dipole interactions. GO expansion shows

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a transition from the interstratification of H2 O monolayer to the accumulation of H2 O multilayers at the interlayer distance of 0.8 nm. The evolution of both hydrogen bonds and H2 O diffusivities suggests the existence of 3 types of H2 O species with different binding states and molecular mobilities. The computed H2 O permeability on the basis of sorption-diffusion theory supports the exceptional H2 O transport capacity in GO membranes.

KEYWORDS: molecular simulation, graphene, gas permeation, surface chemistry, intercalation, adsorption, diffusion

1

Introduction

Recently developed ultrathin graphene oxide (GO) membranes 1,2 show a superior separation performance for a wide range of gas mixtures, including the selective permeation of H2 in H2 /CO2 and H2 /N2 , 3 CO2 in CO2 /N2 , 4 as well as ions in liquid phases, 5 rationalized by a hypothetical molecular sieving mechanism. 6 Water has been identified as a typical species that is capable of forming a few layers of square ice, 7 expanding laminar structure of GO, 8,9 rapidly permeating air-tight GO membrane, 10 effectively altering gas permeance, 4 etc. under ambient conditions. The hydration effect on the structural transformation of GO is largely attributed to its highly polar surface that consists of oxygen-based functional groups, mainly epoxides and hydroxyls, with a variety of concentrations dependent on specific preparation procedures. 11 The presence of oxygen functionalities is also considered to be the prominent factor influencing molecular accessibility and transport for applications of gas separation and energy storage. 12–14 Following the pioneering studies of GO preparation, 15–17 probing the microstructure of GO on the atomic scale becomes increasingly important for the understanding of its aggregation behaviors as well as the related mass transfer phenomena. High resolution transmission electron microscopy (TEM) has been demonstrated as an effective tool to pinpoint the local 2 ACS Paragon Plus Environment

Page 2 of 30

Page 3 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


defects of GO and reduced GO at subnanometer scale. 18,19 Erickson et al. reported that the surface of as-synthesized single layered GO features a continuous, yet disordered, network of oxidized regions, accompanied by isolated graphitic islands of a typical size of 8 nm2 . 18 Upon chemical reduction, the resulting GO monolayer features intact graphene areas with sizes of a few nanometers interspersed with defect areas dominated by clustered pentagons and heptagons. 19 The local structures of GO materials are known to be sensitive to specific fabrication conditions, including oxidizing agent, reaction time, washing procedure, drying and storage, etc. 20–22 Consequentially, the hydration behavior of GO is influenced by the synthesis method, the hydration process, and the surface charge, etc. Within GO, water molecules were found to bond via hydrogen bonds to oxygen atoms of epoxide and OH groups undergoing only localized motions. 23,24 Up to a water loading of 25 weight percent (wt%), the interlayer distance increases from 5.67 to 8 ˚ A, which corresponds to the formation of a water monolayer in the interlayer space of GO. 25 The pronounced impact of hydration is attributed to the presence of the non-homogenous oxidation of GO, where non-oxidized and highly oxidized domains coexist and where a few water molecules trapped between oxidized regions cause the observed large interlayer separations. 26 Using electrostatic force microscopy (EFM), GO sheets were found in a highly heterogeneous state with a random distribution of aggregated surface functionalities and graphitic domains 50 − 100 nm across. 27 Hydrated GO measured by scanning force microscopy (SFM) exhibits a granular morphology of protrusions and valleys with a typical lateral extension around 10 nm and height variation of 0.3 − 0.4 nm. 9 Although GO morphology is known to closely relate to its oxidized surface, the quantitative correlation between experimental findings and nanostructure of GO is still uncertain at this stage. For understanding the unimpeded permeation of water through GO membranes, Nair et al. conducted Molecular Dynamics (MD) simulations of ions 5 and water 10 penetrating the ideal graphene pores to qualitatively interpret the ultrafast molecular transport. The lack of precise GO models and appropriate simulation methods, however, precludes the op-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

portunities of an explicitly comparison of simulation results with experimental data as well as the insight into the molecular process occurring between functionalized surfaces of GO on the nanoscale. Theoretical development of accurate GO models is thus urgently needed to provide mechanistic understandings of graphene materials. Particularly, early studies of Boukhvalov et al. proposed a model structure of GO partially occupied by both epoxide and hydroxyl groups, 28 and also suggested the migration scheme of hydrogen and hydroxyl groups on graphene surface. 29 Similar findings were obtained by Yan et al., who showed energetically favorable configurations of GO containing epoxide and hydroxyl groups in close proximity with each other. 30,31 Liu et al. further conformed that GO models always contain locally ordered structures with good stability at low oxygen coverage. Detailed investigations of graphene oxide models using synchrotron radiation were performed by Saxena et al., who provided experimental evidence of the presence of graphitic regions in graphene oxide. 32 Density function theory (DFT) calculation was applied to quantify binding energies of epoxide/hydroxyl pairs on graphene surface and showed the tendency of agglomeration of these functional groups, implying an inhomogeneous distribution of oxidized and graphitic areas. 11 In addition, five- and six-membered-ring lactols have also been found present in graphite oxide by interpreting the relevant spectroscopic data. 33 Unfortunately, these critical features of GO are often ignored by most molecular simulation works that often employ randomly distributed functional sites for the sake of modeling convenience. 34–39 It has been pointed out that the current theoretical studies conversely suffer from the limit of using simplified molecular systems with a small dimension of a few nanometers, 18 thus missing the critical mesoscopic feature of GO surface functionalities as well as the advance of insight into the nanoscale phenomena of graphene-based applications. In this work, we take the advantage of DFT data to develop realistic all-atom GO models with an emphasis on the oxygen functional group clustering. Taking water as a representative component, we investigate the structural transition of hydrated GO as well as the related H2 O mass transfer phenomenon. Simulations of H2 O adsorption and diffusion are system-


Page 4 of 30

Page 5 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


atically performed and quantitatively analyzed in relation to GO nanostructures. Typical experiments in the aspects of GO preparation and characterization are performed supplementarily for the purpose of modeling validation, but targeting on the exploration of new properties of GO membranes. The necessity of our experimental works is rationalized by the consistence of GO samples used for a serial of characterization measurements that is generally difficult to obtain from the collection of various data sources. The origin of H2 O transport mechanism in GO membranes is addressed in the concluding remarks. In the end, the unique phenomena of H2 O permeation through GO membrane are expected to be deeply revealed in relation to the detailed analysis of GO sorption and diffusion in a quantitative manner. Modeling validation by a direct comparison with selected experimental measurements is facilitated by the use of sorption-diffusion approach and atomic GO models of a representative dimension. The knowledge of transport mechanism attributed to the superior properties of GO membrane is believed to be beneficial and referential when designing novel membrane materials for separation applications.

2 2.1

Methods Simulation methods

Following the work of Zhou et al., 11 typical functional groups, i.e. epoxide and hydroxyl, were generated on intact graphene surface using kinetic Monte Carlo (KMC) method similar with our previous work. 40 In detail, a given number of hydroxyls and epoxides were randomly incorporated on the graphene surface without overlaps, meaning each carbon atom can only couple to one oxygen atom with a bond length of 0.15 nm. Surface groups are then allowed to move among unoccupied sp2 carbon sites on both sides of graphene layer in the way that the total energy of epoxide/hydroxyl pairs, as defined by DFT calculations of Zhou et al., 11 is minimized in a Markov chain process. For a variety of GO oxidization levels, the O/C ratio, denoted here as rOC , was chosen in the typical range of 0.3 − 0.6, and the fraction of epoxide 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

groups over total oxygen functionalities was fixed at a typical value of 0.3. 11 It is known that GO membranes are typically prepared by solution processing, where stacking of GO layers can be expected to be random. In accordance, bilayered GO was generated layer-by-layer following the same modeling procedure as the single GO layer. No structural relaxation was performed for the fixed GO bilayers. The coordinates of oxygen groups between GO layers are therefore independent and the distributions of oxidized domains are not correlated. For structural characterization, GO layers were built with a large size of 34.08 × 39.35 nm2 in the X-Y plane. Periodic boundary conditions were applied. Except otherwise specified, all simulations were performed on the basis of 10 equivalent GO samples to reduce statistical errors. Validation of KMC simulation was achieved by comparing the surface structure of GO models with TEM measurements on the scale of nanometers. Adsorption isotherms of H2 O in GO were calculated using grand canonical Monte Carlo (GCMC) method. The distribution of H2 O was visualized and analyzed from GCMC simulation that is expected to generate the identical statistic results to the molecular configurations in MD simulations. A typical GCMC simulation consists of at least 2 × 106 cycles. In each cycle, trial moves were attempted to translate, rotate, or insert/remove molecules. The number of trial moves per cycle is equal to the number of molecules with a minimum of 20. Heat of adsorption (∆H) was calculated as a function of H2 O loadings using ClausiusClapeyron equation. 41 For computing efficiency, the dimension of GO layers is chosen to be 8.52 × 9.84 nm2 . Both Lennard-Jones (LJ) and Coulomb potentials were employed to calculate H2 O-GO and H2 O-H2 O interactions. The LJ parameters of carbon atom are α = 0.34 nm and = 0.2328 kJ mol−1 taken from GROMACS package. 42 The interaction parameters between hydroxyl/epoxide and H2 O were taken from Stauffer et al. 43 H2 O molecules were simulated using TIP3P model that has been widely employed for investigating adsorption phenomena. 44 The Lorentz-Berthelot combining rule was applied to derive the host-guest LJ parameters. The intermolecular LJ potentials are truncated and shifted at a cut-off distance of 1 nm, beyond which the LJ potential is diminished. 41 The electrostatic potential


Page 6 of 30

Page 7 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


is computed using Ewald summation with a cut-off of 1 nm. Both H2 O and GO were kept rigid during all simulations. The use of rigid GO model is valid to certain extent in that the curvature and self-folding on the same nanoscale as our GO models are less important for the well-defined laminar structure of thin GO membrane fabricated experimentally. Even though the simplified model of GO sheets brings about the convenience on characterizing GO structures and improving computing efficiency, the sorption and dynamic properties of H2 O derived from our simulation are expected to be overestimated slightly due to the neglecting the blocking of interlayer gaps caused by the local bending of rugged GO surface. Moreover, GO models of both layer flexibility and large dimension, e.g. on the order of µm, can be beneficial when examining the impacts of GO folding, pin hole, percolation threshold and so forth using multiscale modeling technique. The NVT MD simulations were conducted to calculate the self diffusion coefficient Dself of H2 O in the slit pore of GO. Temperature is only coupled to H2 O molecules using NoséHoover thermostat. A time step of 2 fs is used to integrate equations of motion. The initial configuration was generated by inserting a given number of H2 O molecules between GO layers with a dimension of 17.04 × 19.68 nm2 . The system was equilibrated for 10 ns, followed by a production run of 10 ns. During this period, coordinates of gas molecules were collected every 20 fs. The mean-square displacements of the center of mass of H2 O were calculated and averaged in both X and Y axises, meaning that the movement of H2 O penetrating through GO layers along the Z axis was not considered. The reason for this is that lateral diffusion along GO slit pores is expected to play a dominant role in the permeation process owing to the high tortuosity imposed by the large size ratio between GO sheet and interlayer distance. Self diffusion coefficients were determined by the linear regression of the mean-square displacement curves over a characteristic time span up to 2 × 106 fs. In addition, the expansion of GO layers upon hydration was simulated using NVT MD method. A given number of H2 O molecules were firstly inserted between two parallel GO layers. The whole system is then relaxed under the constraint that the movement of GO bilayers was only allowed in



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the Z direction. With an increased loading of H2 O, the equilibrated interlayer distances of GO were recorded. All MD simulations were performed using Lammps software. 45 The H2 O permeability was calculated using the classic sorption-diffusion theory, 46 which is the product of self diffusivity and solubility derived from adsorption isotherms. In general, the sorption-diffusion approach assumes a three-step permeation process of adsorption, diffusion and desorption of penetrant components from membranes. The quantification of gas sorption and diffusion using GCMC and MD simulations allows a reliable estimation of permeation data, especially when using the sophisticate models of membrane materials on the nanoscale.

2.2

Experimental methods

Pristine graphite flake (GF) was oxidized to graphite oxide using a modified Hummers’ method that is currently the most commonly used approach. 20,47 In particular, the Hummers’ method involves in three distinct independent steps, including conversion of graphite into a stage-1 graphite intercalation compound by H2 SO4 , formation of oxidized graphite via the diffusion of oxidizing agent KMnO4 into the preoccupied graphite galleries, and the transformation to conventional GO after exposure to water involving hydrolysis of covalent sulfates and loss of all interlayer registry. 48 In detail, GF sample was received from Alfa Aesar, having a median particle size 7−10 µm. It is noted that the dimension of GF particles is slightly smaller than these of 10 − 40 µm used in some other studies, 27,49,50 and the severe oxidation of GF may even decompose GO into fragments as small as mellitic acid. After intensive purification, the as-synthesized samples of GO do show the wrinkled layer structures on the scale of micrometers as evidenced in our previous SEM measurement. 47 The additional benefit from small size of GO aggregates is the acceleration of GO hydration measurements due to the less tortuous H2 O diffusion path within GO aggregates. 2 g GF was dispersed in 50 ml 98% sulphuric acid (Aldrich) in ice bath. Then, 9 g KMnO4 (Aldrich) was slowly added into the slurry with mild stirring. It is known that, compared to graphite power, at least 4 equivalent weight of KMnO4 is required to achieve the complete oxidization of graphite. 20 8 ACS Paragon Plus Environment

Page 8 of 30

Page 9 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The reaction mixture was stirred under room conditions for 4 h. To terminate the oxidation reaction, 100 ml water was slowly poured into the slurry, followed by dripping 30% H2 O2 until bubbling stops. The slurry was diluted using another 300 ml water and stored still overnight to obtain the brownish yellow precipitate. The precipitate was repeatedly washed using 1 mol L−1 HCl and centrifuged at 10000 rpm until SO2− 4 is not detectable using BaCl2 solution. Washing using acetone was then repeated until its pH reaches neutral. 22 Finally, GO was washed with DI water and dried overnight at 40◦ C. Typical characterization measurements were performed using consistent GO samples and analyzed for quantitative modeling validation. Specifically, as-synthesized GO materials were characterized by X-ray photoelectron spectroscopy (XPS, PHI Quantera Scanning X-ray Microprobe), and thermogravimetric and differential scanning calorimetry analysis (Mettler Toledo TG-DSC). Adsorption isotherms of H2 O were measured by dynamic vapor sorption (DVS advantage, micromeritics). The DVS measurement was performed in a moderate temperature range of 300 − 320 K in consideration of the weak thermal stability of surface groups on GO. For DVS, the partial pressure of water in carrier gas nitrogen was precisely adjusted by the use of mass flow control combined with vapor concentration monitoring. A known concentration of water vapor then flows over the GO sample suspended from a recording ultra-microbalance, which measures the change in weight caused by sorption of the water molecules. GO weight around 10 mg was typically used to give the optimum balance between sensitivity, equilibration time and homogeneity. Prior to each adsorption measurement, GO samples were dehydrated at 60◦ C for 2 − 6 h to remove the residual water.

3

Results and discussion

GO sample was prepared using modified Hummers’ method, and its oxidization state was quantified using XPS measurement. The XPS C 1s spectrum (Figure 1e) indicates a ratio of 2 between unoxidized sp2 carbon, namely carbon in aromatic rings (284.7 eV), and sp3



10 nm

(b)

(c)

Intensity [a.u.]

(a)

282

C 1s

283

284

285

287

286

288

Intensity [a.u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 30

528

289

290

O 1s

529

530

531

532

533

534

535

Binding energy [eV]

(d)

(e)

Simulated O/C ratio rOC Graphitic area [%] Disorder area [%] Charge density [C m−2 ]

0.3 61 39 0.79

0.4 48 52 1.06

0.5 35 65 1.32

0.6 22 78 1.58

(f)

Figure 1: Surface distribution of oxygen functionalities on GO monolayer. (a-d) Simulation snapshots of oxygen functionalities on GO monolayer with different O/C ratios of (a) 0.3, (b) 0.4, (c) 0.5, and (d) 0.6. The dimension of simulated GO sheet is 34.08 × 39.35 nm2 . The carbon framework is presented as a green network. Oxygen atoms are red beads, and hydrogen is tiny white bead. (e) XPS C 1s and O 1s spectra of as-synthesized GO samples. The 2 spectra were typically fitted with 3 Gaussian-Lorentzian curves. For the XPS C 1s spectrum, the red peak corresponds to carbon species of C-C, C-H bonds and C vacancies; the blue peak represents carbon involved in epoxide and hydroxyl groups; the green peak is assigned to carbonyl species. For the XPS O 1s spectrum, the peaks are assigned to C=O (red), C-O (blue), and phenolic groups (green), respectively. (f) Simulated surface properties of GO as a function of O/C ratios. The relative standard deviation is less than 2%. 10 ACS Paragon Plus Environment

Page 11 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


carbon bonded to epoxide/hydroxyl groups (286.7 eV) and a small amount of carbonyl species (288.3 eV). While the XPS O 1s spectrum can be interpreted by 3 peaks, i.e. a prominent C=O peak at 531.1 eV, the C-O peak at 532.0 eV, and a weak phenolic peak at 533.4 eV. 51 Our overall O/C ratio is estimated to be 0.34 that is considered to be one of the typical values in the range up to 0.6 as reported previously. 21 This is taken as a guidance for the selection of simulation parameters. In accordance, GO sheets with a wide range of O/C ratios of 0.3 − 0.6 were simulated (Figure 1a-d). From simulation snapshots, it is observed that in the same oxidized region, hydroxyl and/or epoxide groups tend to iteratively co-locate on opposite sides of GO layers as a result of the energetically favorable configurations of hydroxyl/epoxide dimers deduced from DFT calculations, 11 and the oxidized area increases from 40% at rOC = 0.3 to 80% at rOC = 0.6 corresponding to the growth from isolated oxidized islands (Figure 1a) into a fully connected network (Figure 1d). In reality, the simulated variation of concentration and configuration of surface groups is indeed related to the different GO samples prepared under a variety of subtle reaction and post treatment conditions. 21,23,52 As an example, a heavily oxidized GO sample prepared by Erickson et al. has a fraction of 16% graphitic area and 82% high contrast disordered region, 18 in line with our GO model of rOC = 0.6 (Figure 1f). It was observed that graphitic islands surrounded by a continuous network of functional groups featuring irregular shapes and sizes of 1 − 6 nm2 , agreeing quantitatively with that of 1.5 − 6.2 nm in our simulation (Figure 1d). In addition, the existence of single functional sites, primarily epoxides, in the mega-stable state is conformed by both our GO models and aberration-corrected TEM analysis. 18 Water uptake in GO slit pores strongly depends on the complicated force field jointly imposed by neighboring GO surfaces. Despite the irregularity of the oxidized domains, the internal configuration of GO slit pores can be generally classified into 3 categories according to the surface species, namely carbon-carbon (C-C), carbon-oxygen (C-O) and oxygenoxygen (O-O) (Figure 2a). For surface polarity, DFT calculations estimated that sp3 carbon holds a positive partial charge of about 0.18 e, while oxygen atoms are electronegative with



C-O

C-C

O-O

Oxygen funtionalities Carbon network

Type 1

Type 4

Type 6 + -

+ +

Type 5

Type 2

+ + -

+ Type 3 + + (a) 70 rOC=0.3 rOC=0.4

60

Occupancy [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 30

rOC=0.5

50

rOC=0.6

40 30 20 10 0

1

2

3

4

5

6

GO type

(b)

(c)

(d)

Figure 2: Characteristic domains of bilayered GO models. (a) The definition of GO domains. The index denotes the nature of GO interlayer space to be C-C between intact graphene surfaces, C-O between graphene and oxidized surface, and O-O between oxidized surfaces. Surface charges are marked in each figure. The color scheme is cyan for carbon, red for oxygen, and white for hydrogen. (b) The fraction of as-defined GO domains. The mean values and standard deviations were obtained from 5 equivalent GO samples with a dimension of 34.08 × 39.35 nm2 . (c) The local structure of a typical bilayered GO sample with a O/C ratio of 0.4 and a dimension of 8.52 × 9.84 nm2 . (d) The corresponding mapping of C-C (cyan), C-O (blue) and O-O (red) domains.


Page 13 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Qepoxide = −0.36 e and Qhydroxyl = −0.57 e, respectively. 43 These partial charges result in a surface charge density of 1.32 C m−2 for simulated GO with rOC = 0.5 (Figure 1f), close to 0.06 − 1.0 C m−2 measured by EFM. 27,53 In topological view, GO slit pores can be further differentiated to 6 types as illustrated in Figure 2a. Given the fast decay of dipole-dipole potential, it is estimated that negligible electrical fields in bilayered GO pores of type 1, 3 and 6 due to the counteraction of opposite dipole moments, moderate force fields in pores of type 2 and 4, and the strongest in type 5 domain contributed by aligned dipoles. Such a characterization of GO framework is possibly consistent with the 3 types of domains that were experimentally found responding differently upon hydration, namely protrusions including water molecules within GO layers at low humidity, new protrusions of roughly 0.3 nm attributed to water molecules intercalating between GO layers, and protrusion lower than 0.3 nm formed by water incorporating into the GO layers. 9 Using a large set of equivalent GO samples, fractions of as-defined 6 types of GO domains were quantified (Figure 2b). It shows major occupancies of type 1 (C-C), type 4 (C-O) and type 6 (O-O) domains in the absence of 2, 3 and 5 types due to the tendency of functional groups sitting on both sides of GO sheets. Further increase of the oxygen content leads to the growth of type 6 domains accompanied by the suppression of both type 1 and type 4 areas. The visualized distribution of C-C, C-O, and O-O domains in a bilayered GO model in simulation (Figure 2c-d) show the overlapping pattern of functionality agglomerates originating, yet different, from the surface property of single GO layers. To compare binding strengths of H2 O in as-defined GO regions, heats of adsorption (∆H C−C , ∆H C−O and ∆H O−O ) at zero coverage were calculated (Figure 3). By fixing the interlayer distance of GO, the adsorption energy of a single H2 O molecule was computed and recorded to show the impact of slit pore size and surface chemistry in Figure 3a-b. In general, the electroneutral C-C domain possesses the lowest LJ potentials around 10 kJ mol−1 when compared to those of oxidized ones that level off at surprisingly high values of 80 − 100 kJ mol−1 . The difference of calculated H2 O binding energies is strongly dependent



120

140 C-O O-O

100

Coulomb energy [%]

-1

Heat of adsorption [kJ mol ]

80 60 C-C C-O O-O

40

120

100

80

20 0

0.6

0.65

0.7

0.8

0.75

60

0.9

0.85

0.64

0.72

0.68

Interlayer distance [nm]

0.76

0.8

0.84

0.88


(a)

(b) 100

-1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 30

C-O O-O

90

80

70

60

50

0

1

2

3

4

5

6

-2

Loading [molecules nm ]

(c)

Figure 3: Simulated heats of adsorption of H2 O in as-defined GO domains. (a) Heats of adsorption of H2 O at zero coverage as a function of interlayer distance. (b) The relative fraction of Coulomb potentials. (c) Heats of adsorption as a function of H2 O concentration in GO with an interlayer distance of 0.8 nm. Mean values and standard deviations were obtained using 10 equivalent GO samples with a dimension of 2.1 × 2.5 nm2 . The units of H2 O loading is defined as the number of H2 O over the size of GO layer.


Page 15 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


on the distribution oxygen groups, implying the essential role of surface charges played in GO interaction with polar adsorbate H2 O. Indeed, electrostatic potentials account for 80 − 90% of total energies, given that the relative fraction is defined as the Coulomb energy divided by the total energy of the host-guest system. It is noted that a fraction over 100% was reached when LJ interaction of H2 O-GO became repulsive in narrow GO pores (Figure 3b). Intrinsic interlayer distances corresponding to the maximum ∆H are 0.7 nm for C-O domain, and 0.8 nm for O-O domain because of the bulge of functional groups out of graphene basal plane, disturbing the formation of H2 O monolayer. Moreover, the decrease of ∆H (Figure 3c) suggests that even for the same type of GO domain, surface heterogeneity still exists due to the interspersion of hydroxyl/epoxide groups. For experimental validation, heats of adsorption of H2 O in GO were calculated using TGDSC measurements (Figure 4a-b). The initial water content of GO samples was estimated to be 20 − 25 wt% at the ambient conditions of 30 ± 10% relative humidity and 300 K. At a low scan rate of 2 K min−1 , pronounced decomposition of oxidized GO takes place at the temperature of 100◦ C that serves as an upper limit for H2 O desorption measurement. Strictly speaking, the endothermic process below this threshold reflects the slow release of H2 O accompanied by minor decomposition of oxygen functionalities of GO. At a constant temperature of 60◦ C, the heat of adsorption is roughly determined to be 40 kJ mol−1 (Figure 4b), and the desorbed H2 O is considered to be the ones weakly binding inside GO. Both of the broad DSC peak (Figure 4a) and slow equilibration (Figure 4b) reflect the impeded water desorption exerted by the strong affiliation of H2 O-GO. As a comparison, our calculated ∆H is in the range of 50 − 90 kJ mol−1 (Figure 4c), slightly higher than our DSC data. The discrepancy can be caused by the condensed water contained in meso/macropores between GO stacks, or the exothermic effect of functionality decomposition. Moreover, the calculated ∆H of 80−90 kJ mol−1 at low H2 O loadings corresponds to the adsorption strength between C-O and O-O domains, and further H2 O uptake results in the decrease of ∆H eventually to that of the hydrophobic C-C domain. The high H2 O sorption enthalpy of GO is comparable


Weight ppercent [%]

100 80 60 40

Page 16 of 30

100 96 92 88 -0.2

-1

Heat flow [W g ]

2.5

-1

Heat flow [W g ]

2 1.5 1 300

350

400

450

500

-0.3 -0.4 -0.5 -0.6

550

0

10

20

Temperature [K]

30

40

50

60

Time [min]

(a)

(b) ∆H0

90

O-O

-1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Weight percent [%]


∆H0

C-O

80 O-O

∆H

70

C-O

∆H

60

50

0

1

2

3

4

5

6

-2


(c)

Figure 4: Experimental validation of calculated H2 O adsorption enthalpies in GO. (a) TGDSC analysis of GO sample heating up to 573 K at a scan rate of 2 K min−1 . (b) TG-DSC analysis of GO sample at a consistent temperature 333 K. The temperature rise is finished in 3.5 min. (c) Heats of adsorption as a function of H2 O loading. The relative standard deviation is less than 5%. The bilayered GO has a O/C ratio of 0.4, an interlayer distance of 0.8 nm, and a dimension of 8.52 × 9.84 nm2 .


Page 17 of 30

to many water-adsorbent materials, e.g. acidic zeolites, 54 metal organic frameworks, 55 etc., making it a promising material for water adsorption. 10

-2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8

rO/C=0.3 rO/C=0.4 rO/C=0.5

6

4

2

0 -5 10

-4

10

-3

-2

10

10

-1

10

0

10

Pressure [kPa]

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

Figure 5: Formation of H2 O monolayer in a bilayered GO sample at 300 K. (a) Adsorption isotherms of H2 O in GO with various O/C ratios. The standard deviations are smaller than symbols. (b-k) Molecular configurations of H2 O in reference to as-defined GO domains. H2 O loadings are (b) 1.08, (c) 1.22, (d) 2.07, (e) 2.90, (f) 3.84, (g) 4.83, (h) 6.14, (i) 7.09, (j) 7.85, and (k) 8.80 molecules nm2 . The bilayered GO has a O/C ratio of 0.4, an interlayer distance of 0.8 nm, and a dimension of 8.52 × 9.84 nm2 . The color scheme is identical to the one used in Figure 2d. The decrease of ∆H possibly suggests the H2 O occupancy order of O-O → C-O → C-C



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

domains. Conforming this hypothesis necessitates simulation snapshots of adsorbed H2 O molecules in GO. Firstly, adsorption isotherms (Figure 5a) show the positive effect of O/C ratio on H2 O loading owing to the strong adsorption capacity of oxygen functionalities. The intermediate molecular configurations are visualized in reference to as-defined GO domains (Figure 5b-k). It is interesting to see that the accommodation of H2 O shows a close correlation with the distribution of 3 GO domains. To be specific, H2 O uptake in GO slit pores can be divided into 4 stages. (1) At low loading, H2 O molecules tend to preferentially occupy the O-O domain due to the strong interaction (∆H ≈ 90 kJ mol−1 ) with hydroxyl/epoxide (Figure 5b-c). (2) Further loading results in the sparse distribution of H2 O at C-O domains accompanied by H2 O compaction in the O-O domain (Figure 5d-e). (3) After saturation in the O-O domain, H2 O molecules attempt to flood the C-O domain. Meanwhile, water bridges across the C-C domain form between adjacent C-O or O-O domains, leading to the occupancy of H2 O in hydrophobic pores (Figure 5f-h). Thus, H2 O coverage in the C-C domain is mainly attributed to strong hydrogen bonds among H2 O molecules rather than LJ interactions with sp2 carbon atoms. This assessment is consistent with the fact of much weaker water-surface interaction than the interaction between water molecules drawn from the experimental observation of 2D ice formation inside hydrophobic nanocapillaries of graphene under ambient conditions. 7 (4) At the high humidity (Figure 5i-k), water bridges widen, merge and spread out eventually to a H2 O monolayer through the entire volume of GO slit pores. GO expansion then take place upon further water condensation. Simulated adsorption isotherms of H2 O were validated using DVS measurements (Figure 6a). It is noted that water contents at low relative humidity cannot be accurately measured due to the strong H2 O affinity of GO. The desorption curves are identical to the adsorption ones, and thus are not shown in Figure 6a. In general, our results agree well with experimental data under moderate relative humidity, while certain discrepancy was noted at high H2 O pressure. This is attributed to the use of rigid GO models with fixed interlayer distance that may underestimate H2 O loadings by ignoring the enlargement of interlayer volume dur-


Page 18 of 30

Page 19 of 30

Relative humidity

Exp. at 320 K Sim. at 320 K

40 30

1.2


10 0 Exp. at 310 K Sim. at 310 K

40 30 20 10 0

Exp. at 300 K Sim. at 300 K

40 30

1.1

0.6

0.8

1

De Boer et al. Wagner et al. Lerf et al. Exp. this work Sim. this work

1

3 layers

2 layers

Expansion

0.9 0.8

1 layer

0.7

Interstratification

0.6

20 10 0

0.4

1.3

20

Weight percent [%]

0.1

0.2

0.3

0.4

0.5

0.5 0

0.6

2

4

8

6

10

12

14

-2


Relative humidity

(a)

(b)

0.07 d=0.79 nm d=0.83 nm d=0.96 nm d=1.08 nm d=1.21 nm

0.06 0.05

Fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.04 0.03 0.02 0.01 0 -0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

Z coordinate [nm]

(c)

Figure 6: The GO expansion. (a) Adsorption isotherms of H2 O as a function of relative humidity in comparison with DVS data at various temperatures. The standard deviations are smaller than the symbols. (b) The interlayer distance of GO sheets as a function of H2 O content at 300 K. The standard deviations of simulation data are smaller than the symbols. The number of H2 O layers formed within GO interlayer space was estimated. The experimental data are taken from Ref. 24 and Ref. 56. (c) The corresponding spatial distribution of water molecules in bilayered GO (X-Y plane) along Z axis. The GO sample has a C/O ratio of 0.3 and a dimension of 17.04 × 19.68 nm2 . ing GO expansion. With this concern, GO expansion was quantitatively studied using MD simulation (Figure 6b). Starting from the dry GO with an interlayer distance of 0.56 nm, the transition of GO laminar structure was found to undergo two distinct phases, namely interstratification and expansion. (1) At low humidity, H2 O molecules tend to intrude into the slit pore of GO and continuously occupy the O-O, C-O and C-C domains forming a



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

single water film. In accordance, the interlayer distance of GO firstly increases to 0.75 nm, and then undergoes a gradual transition to a value of 0.8 nm as commonly reported by other studies. 24 (2) Following the formation of H2 O monolayer at about 6 molecules nm−2 , a linear expansion of GO at a higher rate takes place due to the accumulation of H2 O multilayers. The simulated two-phase transition of GO layers can also be recognized from experimental data of GO hydration, occurring around the relative humidity of 0.7 − 0.8. 24,56 The corresponding layered structures of H2 O are characterized (Figure 6c). During interstratification (d = 0.75 − 0.8 nm), the adsorbed H2 O molecules tend to form a monolayer, while an additional layer appears at a distance of 1 nm. A further expansion to 1.2 nm allows the accommodation of 3 water layers. Continuous hydration will eventually result in complete exfoliation of GO sheets occurs as experimentally obtained by merging GO into liquid water. In all cases, a portion of H2 O molecules keep binding tightly to the oxidized surface of GO at a distance of 0.35 nm. It is noted that despite the complicated stepwise process of H2 O loading, the assembling behavior of water within GO layers shows a certain extent of similarity with nanoconfined water between defect-free graphene sheets, which forms square ice having a high packing density with a lattice constant of 0.283 nm and capable of assembling into bilayer and trilayer crystallites. 7 In relation to the expandable framework of GO membrane, the dynamics of H2 O is quantified in terms of self diffusion coefficients (Figure 7a-b). The histogram distribution of Dself is analyzed in reference to an imperial threshold of Dself = 10−8 cm2 s−1 below which H2 O molecules are considered to be immobile. It was found that a trace amount of H2 O molecules fall in this limit, indicating H2 O trapped by the strong binding with surface groups. Beyond that, Dself follows normal distribution, and the mean value increases along with the interlayer distance of GO. Water in bilayered GO features multitype diffusion from retarded movement in the O-O domain to the frictionless flow in the C-C domain. In GO slit pores larger than 0.8 nm, the rapid motion is attributed to the highly mobile H2 O molecules diffusing between 2 bonded water layers that efficiently screen the polarity of oxidized GO surface. For narrow


Page 20 of 30

Page 21 of 30

-4

10 O-O

C-O

D

C-C

D

D

0.12

0.08

Bulk water

2 -1

Self diffusivity [cm s ]

0.16

Fraction

d=0.75 nm d=0.77 nm d=0.79 nm d=0.96 nm d=1.21 nm

0.04

-5

10

-6

10 0 -8 10

-7

10

-6

-5

10

10

-4

10

0.7

2 -1

0.8

0.9

1

1.1

1.2


Self diffusivity [cm s ]

(a)

(b) 0.12

2

0.1

1.6

Normalized Hbonds

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.08 1.2 0.06 0.8 0.04 H2O-GO H2O-H2O

0.4

0

0.8

0.9

1

1.1

0.02

1.2

0


(c)

Figure 7: Dynamics of H2 O molecules in bilayered GO at 300 K. (a) Distribution and (b) mean values of self diffusion coefficients of H2 O as a function of interlayer distance. The C/O ratio of GO is 0.3, and the dimension 17.04 × 19.68 nm2 . (c) The number of hydrogen bonds as a function of interlayer distance. The number of hydrogen bonds was normalized by the total number of oxygen functionalities. pores (d < 0.8 nm), the improvement of H2 O transport is indeed attributed to H2 O molecules moving in the hydrophobic C-C domains, which only experience a certain degree of internal tortuosity caused by the obstruction of bonded water clusters. H2 O species can thus be classified into 3 types according to their dynamic states, i.e. free water separated from GO surface by bonded water layers, bonded water tightly coupled onto oxidized surface, and confined water near the hydrophobic surface. In Figure 7b, it is found that log(Dself ) increases linearly with the interlayer distance, obeying the Arrhenius relationship. At a distance of 21 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.8 nm, the slope of log(Dself ) is significantly reduced owing to the emergence of free water upon saturation of confined water. It is important to see that above 1 nm, Dself even exceeds the simulated and validated Dself of bulk water, providing another evidence supporting the ultrafast H2 O permeation through GO membrane. 10 In addition, the retardation effect of GO functionalities on H2 O mobility was analyzed in terms of hydrogen bonds (Figure 7c). The number of H2 O hydrogen bonds again shows a transition around the interlayer distance of 0.8 nm. Below this value, increases of both H2 O-GO and H2 O-H2 O hydrogen bonds can be assigned mainly to the continuous coverage of monolayer H2 O on oxidized surfaces, and water bridges over the hydrophobic area, respectively. Beyond that, the formation of additional water film between expanded GO layers leads to the increase of H2 O-H2 O hydrogen bonds, while the number of H2 O-GO hydrogen bonds remain constant. In terms of water permeation, Nair et al. attempted to interpret the unique phenomenon of water transport in GO on the basis of both the classical flow equation and MD simulation, and qualitatively concluded a low-friction flow of a monolayer of water through two-dimensional capillaries formed by closely spaced graphene sheets. A direct calculation and comparison of water permeation data were, however, not achieved. By taking the advantage of the realistic GO models and sorption-diffusion method, we explicitly quantify the correlation between GO surface structure and dynamic behavior of water. Particularly, our computed H2 O permeability shows a high value on the order of 10−4 g cm−1 s−1 bar−1 , reasonably agreeing with that of 10−6 g cm−1 s−1 bar−1 estimated in water evaporation experiment. 10 Apart from the experimental uncertainty, the deviation can be caused by neglecting the longitudinal motion of H2 O penetrating through gaps near the edges of GO layers, as well as the flexibility of aggregated GO sheets in our MD simulation. Nevertheless, it can be conformed that both results suggest the superior permeation capacity of GO membranes originates from its strong water affinity and highly heterogeneous framework.


Page 22 of 30

Page 23 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

Conclusions

All-atom models of GO with agglomerated hydroxyl/epoxide groups were developed in line with DFT data and experimental characterization of GO surfaces. Different topologies of surface groups were simulated and compared to GO samples prepared under subtle synthetic conditions. Inside the slit pores of GO membranes, 3 types of interlayer domains are identified to have different combination of internal functionalities and degrees of hydrophilicity. Adsorption enthalpies and isotherms of H2 O are computed and validated using TG-DSC and DVS data. The occupancy of H2 O strictly follows the hydrophilicity sequence of as-defined GO domains due to the strong electrostatic interactions with oxygen functionalities. The structural transition of GO was found to occur at the interlayer distance of 0.8 nm upon hydration. Below this value, water uptake leads to the interstratification of H2 O monolayer and the increase of both H2 O-GO and H2 O-H2 O hydrogen bonds. Above that, H2 O multilayers are formed, while the number of H2 O-GO hydrogen bonds remains constant. In terms of the binding state and molecular mobility, 3 types of water species can be identified as bonded water that binds tightly to oxygen functionalities at low humidity, confined water with the movement obstructed by bonded water clusters, and free water well separated from oxidized surfaces of expanded GO. The computed H2 O diffusivities increase along with the interlayer distance of GO, attributing to the sequential loading of bonded water, confined water and free water. Particularly, the accumulation of free water in the enlarged GO pores results in the H2 O diffusivity exceeding that of bulk water under sufficient hydration. On the basis of the sorption-diffusion theory, our calculation shows a high H2 O permeability, well supporting the exceptional water transport through GO membranes reported experimentally.

Acknowledgement This work was supported by the open funding project from State Key Laboratory of Offshore Oil Exploitation of China. The authors acknowledge the financial supports from National 23 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Natural Science Funds for Distinguished Young Scholar (Grant No: 21303168), and Science Foundation of China University of Petroleum, Beijing (Grant No: 2462014YJRC009).

References (1) Liu, G.; Jin, W.; Xu, N. Graphene-Based Membranes. Chem. Soc. Rev. 2015, 44, 5016–5030. (2) Huang, L.; Zhang, M.; Li, C.; Shi, G. Graphene-Based Membranes for Molecular Separation. J. Phys. Chem. Lett. 2015, 6, 2806–2815. (3) Li, H.; Song, Z.; Zhang, X.; Huang, Y.; Li, S.; Mao, Y.; Ploehn, H. J.; Bao, Y.; Yu, M. Ultrathin, Molecular-Sieving Graphene Oxide Membranes for Selective Hydrogen Separation. Science 2013, 342, 95–98. (4) Kim, H. W.; Yoon, H. W.; Yoon, S.-M.; Yoo, B. M.; Ahn, B. K.; Cho, Y. H.; Shin, H. J.; Yang, H.; Paik, U.; Kwon, S.; Choi, J.-Y.; Park, H. B. Selective Gas Transport Through Few-Layered Graphene and Graphene Oxide Membranes. Science 2013, 342, 91–95. (5) Joshi, R. K.; Carbone, P.; Wang, F. C.; Kravets, V. G.; Su, Y.; Grigorieva, I. V.; Wu, H. A.; Geim, A. K.; Nair, R. R. Precise and Ultrafast Molecular Sieving Through Graphene Oxide Membranes. Science 2014, 343, 752–754. (6) Mi, B. Graphene Oxide Membranes for Ionic and Molecular Sieving. Science 2014, 343, 740–742. (7) Algara-Siller, G.; Lehtinen, O.; Wang, F. C.; Nair, R. R.; Kaiser, U.; Wu, H. A.; Geim, A. K.; Grigorieva, I. V. Square Ice in Graphene Nanocapillaries. Nature 2015, 519, 443–445. (8) Guo, F.; Kim, F.; Han, T. H.; Shenoy, V. B.; Huang, J.; Hurt, R. H. Hydration-


Page 24 of 30

Page 25 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Responsive Folding and Unfolding in Graphene Oxide Liquid Crystal Phases. ACS Nano 2011, 5, 8019–8025. (9) Rezania, B.; Severin, N.; Talyzin, A. V.; Rabe, J. P. Hydration of Bilayered Graphene Oxide. Nano Lett. 2014, 14, 3993–3998. (10) Nair, R. R.; Wu, H. A.; Jayaram, P. N.; Grigorieva, I. V.; Geim, A. K. Unimpeded Permeation of Water Through Helium-Leak-Tight Graphene-Based Membranes. Science 2012, 335, 442–444. (11) Zhou, S.; Bongiorno, A. Density Functional Theory Modeling of Multilayer “Epitaxial” Graphene Oxide. Acc. Chem. Res. 2014, 47, 3331–3339. (12) Shao, J.-J.; Lv, W.; Yang, Q.-H. Self-Assembly of Graphene Oxide at Interfaces. Adv. Mater. 2014, 26, 5586–5612. (13) Luo, J.; Jang, H. D.; Huang, J. Effect of Sheet Morphology on the Scalability of Graphene-Based Ultracapacitors. ACS Nano 2013, 7, 1464–1471. (14) Huang, Y.; Liang, J.; Chen, Y. An Overview of the Applications of Graphene-Based Materials in Supercapacitors. Small 2012, 8, 1805–1834. (15) Boehm, H. P.; Clauss, A.; Fischer, G. O.; Hofmann, U. Das Adsorptionsverhalten sehr d¨ unner Kohlenstoff-Folien. Z. Anorg. Allg. Chem. 1962, 316, 119–127. (16) Brodie, B. C. On the Atomic Weight of Graphite. Philos. Trans. R. Soc. London 1859, Ser. A 149, 249–259. (17) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339–1339. (18) Erickson, K.; Erni, R.; Lee, Z.; Alem, N.; Gannett, W.; Zettl, A. Determination of the Local Chemical Structure of Graphene Oxide and Reduced Graphene Oxide. Adv. Mater. 2010, 22, 4467–4472. 25 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(19) Gomez-Navarro, C.; Meyer, J. C.; Sundaram, R. S.; Chuvilin, A.; Kurasch, S.; Burghard, M.; Kern, K.; Kaiser, U. Atomic Structure of Reduced Graphene Oxide. Nano Lett. 2010, 10, 1144–1148. (20) Dimiev, A. M.; Tour, J. M. Mechanism of Graphene Oxide Formation. ACS Nano 2014, 8, 3060–3068. (21) Dimiev, A.; Kosynkin, D. V.; Alemany, L. B.; Chaguine, P.; Tour, J. M. Pristine Graphite Oxide. J. Am. Chem. Soc. 2012, 134, 2815–2822. (22) Luo, J.; Kim, J.; Huang, J. Material Processing of Chemically Modified Graphene: Some Challenges and Solutions. Acc. Chem. Res. 2013, 46, 2225–2234. (23) Buchsteiner, A.; Lerf, A.; Pieper, J. Water Dynamics in Graphite Oxide Investigated with Neutron Scattering. J. Phys. Chem. B 2006, 110, 22328–22338. (24) Lerf, A.; Buchsteiner, A.; Pieper, J.; Schöttl, S.; Dekany, I.; Szabo, T.; Boehm, H. Hydration Behavior and Dynamics of Water Molecules in Graphite Oxide. J. Phys. Chem. Solids 2006, 67, 1106–1110. (25) Cerveny, S.; Barroso-Bujans, F.; Alegria, A.; Colmenero, J. Dynamics of Water Intercalated in Graphite Oxide. J. Phys. Chem. C 2010, 114, 2604–2612. (26) Zhou, S.; Kim, S.; Di Gennaro, E.; Hu, Y.; Gong, C.; Lu, X.; Berger, C.; de Heer, W.; Riedo, E.; Chabal, Y. J.; Aruta, C.; Bongiorno, A. Film Structure of Epitaxial Graphene Oxide on SiC: Insight on the Relationship Between Interlayer Spacing, Water Content, and Intralayer Structure. Adv. Mater. Interfaces 2014, 1, 1300106. (27) Kulkarni, D. D.; Kim, S.; Chyasnavichyus, M.; Hu, K.; Fedorov, A. G.; Tsukruk, V. V. Chemical Reduction of Individual Graphene Oxide Sheets as Revealed by Electrostatic Force Microscopy. J. Am. Chem. Soc. 2014, 136, 6546–6549.


Page 26 of 30

Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Boukhvalov, D. W.; Katsnelson, M. I. Modeling of Graphite Oxide. J. Am. Chem. Soc. 2008, 130, 10697–10701. (29) Boukhvalov, D. W. Modeling of Hydrogen and Hydroxyl Group Migration on Graphene. Phys. Chem. Chem. Phys. 2010, 12, 15367–15371. (30) Yan, J.-A.; Xian, L.; Chou, M. Y. Structural and Electronic Properties of Oxidized Graphene. Phys. Rev. Lett. 2009, 103, 086802. (31) Yan, J.-A.; Chou, M. Y. Oxidation Functional Groups on Graphene: Structural and Electronic Properties. Phys. Rev. B 2010, 82, 125403. (32) Saxena, S.; Tyson, T. A.; Shukla, S.; Negusse, E.; Chen, H.; Bai, J. Investigation of Structural and Electronic Properties of Graphene Oxide. Appl. Phys. Lett. 2011, 99, 013104. (33) Gao, W.; Alemany, L. B.; Ci, L.; Ajayan, P. M. New Insights into the Structure and Reduction of Graphite Oxide. Nat. Chem. 2009, 1, 403–408. (34) Medhekar, N. V.; Ramasubramaniam, A.; Ruoff, R. S.; Shenoy, V. B. Hydrogen Bond Networks in Graphene Oxide Composite Paper: Structure and Mechanical Properties. ACS Nano 2010, 4, 2300–2306. (35) Kim, D.; Kim, D. W.; Lim, H.-K.; Jeon, J.; Kim, H.; Jung, H.-T.; Lee, H. Intercalation of Gas Molecules in Graphene Oxide Interlayer: The Role of Water. J. Phys. Chem. C 2014, 118, 11142–11148. (36) Wei, N.; Lv, C.; Xu, Z. Wetting of Graphene Oxide: A Molecular Dynamics Study. Langmuir 2014, 30, 3572–3578. (37) Bagri, A.; Mattevi, C.; Acik, M.; Chabal, Y. J.; Chhowalla, M.; Shenoy, V. B. Structural Evolution During the Reduction of Chemically Derived Graphene Oxide. Nat. Chem. 2010, 2, 581–587. 27 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(38) Wei, N.; Peng, X.; Xu, Z. Understanding Water Permeation in Graphene Oxide Membranes. ACS Appl. Mater. Interfaces 2014, 6, 5877–5883. (39) Jiao, S.; Xu, Z. Selective Gas Diffusion in Graphene Oxides Membranes: A Molecular Dynamics Simulations Study. ACS Appl. Mater. Interfaces 2015, 7, 9052–9059. (40) Ban, S.; van Laak, A. N. C.; Landers, J.; Neimark, A. V.; de Jongh, P. E.; de Jong, K. P.; Vlugt, T. J. H. Insight into the Effect of Dealumination on Mordenite Using Experimentally Validated Simulations. J. Phys. Chem. C 2010, 114, 2056–2065. (41) Frenkel, D.; Smit, B. Understanding Molecular Simulation: From Algorithms to Applications, 2nd ed.; Academic Press: London, 2002. (42) Berendsen, H.; van der Spoel, D.; van Drunen, R. GROMACS: A Message-Passing Parallel Molecular Dynamics Implementation. Comput. Phys. Commun. 1995, 91, 43– 56. (43) Stauffer, D.; Dragneva, N.; Floriano, W.; Mawhinney, R.; Fanchini, G.; French, S.; Rubel, O. An Atomic Charge Model for Graphene Oxide for Exploring Its Bioadhesive Properties in Explicit Water. J. Chem. Phys. 2014, 141, 044705. (44) MacKerell, A. D.; Bashford, D.; Bellott, M.; Dunbrack, R.; Evanseck, J. D.; Field, M. J.; Fischer, S.; Gao, J.; Guo, H.; Ha, S. All-Atom Empirical Potential for Molecular Modeling and Dynamics Studies of Proteins. J. Phys. Chem. B 1998, 102, 3586–3616. (45) Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1–19. (46) Graham, T. On the Absorption and Dialytic Separation of Gases by Colloid Septa. Philos. Trans. R. Soc. London 1866, 156, 399–439. (47) Ban, S.; Jing, X.; Zhou, H.; Zhang, L.; Zhang, J. Experimental and Modeling Study


Page 28 of 30

Page 29 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


on Charge Storage/Transfer Mechanism of Graphene-Based Supercapacitors. J. Power Sources 2014, 268, 604–609. (48) Sorokina, N.; Khaskov, M.; Avdeev, V.; Nikol’skaya, I. Reaction of Graphite with Sulfuric Acid in the Presence of KMnO4 . Russ. J. Gen. Chem. 2005, 75, 162–168. (49) Mermoux, M.; Chabre, Y.; Rousseau, A. FTIR and 13 C NMR Study of Graphite Oxide. Carbon 1991, 29, 469–474. (50) Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Synthesis and Exfoliation of Isocyanate-Treated Graphene Oxide Nanoplatelets. Carbon 2006, 44, 3342–3347. (51) Chen, D.; Feng, H.; Li, J. Graphene Oxide: Preparation, Functionalization, and Electrochemical Applications. Chem. Rev. 2012, 112, 6027–6053. (52) Eigler, S.; Hirsch, A. Chemistry with Graphene and Graphene Oxide — Challenges for Synthetic Chemists. Angew. Chem. Int. Ed. 2014, 53, 7720–7738. (53) Burnett, T.; Yakimova, R.; Kazakova, O. Mapping of Local Electrical Properties in Epitaxial Graphene Using Electrostatic Force Microscopy. Nano Lett. 2011, 11, 2324– 2328. (54) Bolis, V.; Busco, C.; Ugliengo, P. Thermodynamic Study of Water Adsorption in HighSilica Zeolites. J. Phys. Chem. B 2006, 110, 14849–14859. (55) K¨ usgens, P.; Rose, M.; Senkovska, I.; Fröde, H.; Henschel, A.; Siegle, S.; Kaskel, S. Characterization of Metal-Organic Frameworks by Water Adsorption. Microporous Mesoporous Mater. 2009, 120, 325–330. (56) De Boer, J. B.; Doorn, A. B. C. Graphite Oxide. V. The Sorption of Water. Koninkl. Nederl. Akad. Wetenschappen 1958, Ser. B 61, 242–252.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

Graphical TOC Entry Bilayered GO

H2O occupancy O-O C-O

C-C

Carbon

Water bridge

Hydroxyl/epoxide


Insight into the Nanoscale Mechanism of Rapid H2O Transport within

Recommend Documents